Why probably 90% of the questions here are have to be answered with another question?!
you can not login how? whats happening? what do you see? what error logs say? what php version? what kind of server? did it work before? are you sure you typed correct username/pass?
"What this?" - you did something wrong or/and something wrong with your server or/and so on, so on, so on....
See, you did not provide absolutly no usefull information!